Water-rich environments trigger coal instability risks via dynamic energy evolution and microscopic damage mechanisms

Water-rock interaction significantly influences the mechanical behavior and failure characteristics of coal masses; however, the underlying mechanisms governing the transition of failure modes from dry to saturated conditions remain insufficiently understood. In this study, uniaxial compression tests combined with particle flow numerical simulations were conducted to investigate the mechanical response, crack evolution, force-chain characteristics, and failure mechanisms of coal specimens under dry and water-saturated conditions. The mesoscale evolution of cracks and force-chain networks was quantitatively characterized to elucidate the intrinsic relationship between internal damage development and macroscopic failure patterns. The results indicate that water saturation markedly deteriorates the mechanical properties of coal, leading to pronounced reductions in compressive strength and elastic modulus while accelerating damage accumulation. Dry specimens predominantly exhibit conjugate shear failure characterized by X-shaped shear bands and relatively large fragmented blocks. In contrast, saturated specimens are dominated by tensile failure, with axial cracks initiating from the specimen base and propagating parallel to the loading direction, resulting in more severe fragmentation and higher crack density. Analysis of crack hotspots and force-chain distributions reveals that macroscopic failure is governed by the evolution, rupture, and reorganization of internal force-chain networks. In dry specimens, strong force chains effectively sustain localized shear stress concentrations, promoting shear crack coalescence. Water-rock interaction weakens inter-particle bonding and frictional resistance, redistributes internal stresses, and disrupts force-chain continuity, thereby facilitating the initiation and propagation of tensile microcracks. The formation of force chains oriented obliquely to the loading direction is identified as an important precursor to tensile failure. These findings provide new insights into the mesoscale mechanisms of water-induced weakening and failure-mode transition in coal and offer a theoretical basis for the stability evaluation and disaster prevention of water-bearing coal engineering.